The field of the invention is porous silicon formation.
Silicon, in its naturally occurring elemental form, is not light emitting. Silicon may be changed to porous silicon, a modified form of silicon. The unique electronic, morphological, and thermal properties of porous silicon make it useful for a range of applications. Porous silicon may even be light emitting, making it useful in optoelectronics.
In addition to potential applications in silicon-based optoelectronics, porous silicon has been used as an antireflective coating for silicon solar cells. Chemically modified porous silicon may be useful in chemical and biochemical sensing. Porous silicon can serve as an efficient matrix for direct introduction of high mass biomacromolecules in mass spectrometry. In sum, porous silicon is useful in numerous applications and is likely to find many additional uses in the future.
Conventional methods for producing porous silicon are often time-consuming, difficult, or ineffective in producing stable porous silicon structures. Equipment such as a potentiostats and illuminating light sources are required in etching processes of conventional porous silicon production methods. Porous silicon is normally produced by anodic etching, with (n-type) or without (p-type) illumination. In the anodic etch process mobile holes are electrically driven to the silicon-electrolyte interface where they participate in the oxidative dissolution of surface silicon atoms. Spatial anisotropy results from the potential barrier developed at the sharp tips of the evolving structures, which block further hole transport thus preventing further etching and giving rise to the porous structure. Porous silicon has also been made without external bias by chemical etching in HNO3/HF solutions (stain etching), and by photochemical etching.
Stain etching is typically slow (characterized by an induction period), inconsistent in result, unreliable in producing light-emitting porous silicon and is not readily amenable to lateral patterning. Stain etching is mainly used for making very thin porous silicon layers. Recently, it was shown that evaporating and annealing 150-200 nm of aluminum (Al) on Si results in more rapid stain etching. However, the porous silicon produced by this aluminum enhanced stain etching was approximately ten times weaker in luminescence than anodically etched porous silicon of a similar thickness, and the process still exhibits an induction period prior to commencement of etching. See, D. Dimova Malinovska et al., xe2x80x9cThin Solid Filmsxe2x80x9d, 297, 9-12 (1997). It has also been reported that Pt could be deposited electrochemically from a Pt (IV) solution onto Si during etching to produce light-emitting porous silicon, although it proved difficult to control the applied potential to affect both silicon etching and Pt deposition simultaneously. See, P. Gorostiza, R. Diaz, M. A. Kulandainathan, F. Sanz, and J. R. Morante, J. Electroanal. Chem. 469, 48 (1999).
Thus, there is a need for an improved method for forming porous silicon. It is an object of the invention to meet that need.
The present method produces porous silicon (PSi) with tunable morphologies and light emitting properties. In the method of the invention a thin discontinuous metal layer is deposited on a silicon surface. Preferred metals are Pt for strongly light-emitting PSi, Au for PSi with smooth morphology. It is important that the deposited layer be sufficiently thin that it forms a discontinuous film, thereby providing access of etchant species to the silicon surface in the area of the deposited metal. The surface is then etched in a solution including HF and an oxidant for a brief period, as little as 2 seconds to as much as 60 minutes. A preferred oxidant is H2O2. Morphology and light emitting properties of porous silicon can be selectively controlled as a function of the type of metal deposited, Si doping type, silicon doping level, and etch time. Electrical assistance is unnecessary during the chemical etching of the invention.